A known vehicle suspension system includes a thrust generating device for regulating a relative displacement between a sprung member and an unsprung member by generating a thrust force, and a damper connected in parallel with the thrust generating device to produce a damping force according to a relative velocity between the sprung member and the unsprung member. See JP2010-253989A, for instance. In the prior art disclosed in JP2010-253989A, a sprung member control command value is computed according to a skyhook control (ride quality oriented control) which brings the sprung member velocity to zero with the aim to control the vertical vibration of the sprung member, and a unsprung member control command value is computed according to a damping control (road tracking oriented control) with the aim to improve the road tracking performance of the wheel. The control command value for the thrust generating device is computed by adding the sprung member control command value to the unsprung member control command value. In such a control process, the sprung member control command value and the unsprung member control command value that are determined may cancel out each other. Therefore, in the prior art disclosed in JP2010-253989A, the unsprung member velocity used for computing the unsprung member control command value is processed by a control property adjustment filter to control the damping force control by bringing the gain of the filter close to zero when the vibration frequency of the unsprung member is lower than the unsprung member resonance frequency. Thereby, in the region of the sprung resonance frequency which is lower than the unsprung member resonance frequency, the sky hook control is dominant and the ride quality is given a higher priority.
However, according to the control process disclosed in JP2010-253989A, because the damping force control is restrained in a frequency range lower than the unsprung member resonance frequency range, an appropriate measure is required to be taken in a situation where a transient input, instead of a vibratory input, is applied to the wheel (unsprung member) such as when the wheel rides over a short bump. Particularly in the case of an electromagnetic damper (electromagnetic actuator) which produces a drive force and a damping force with an electric motor, when the damping force control is restrained, a hydraulic damper is required to be provided in parallel with the electromagnetic damper. When such a hydraulic damper is used, the electromagnetic damper is required to produce a drive force for overcoming the damping force of the hydraulic damper so that the size of the suspension system is required to be increased, and the control mechanism is made more complex.